The present invention generally relates to integrated circuits containing data storage circuits and functional circuits all of which are coupled to other components, external to the integrated circuit, through signal interface pins. A small number of these signal interface pins are coupled only to the functional circuits and are used to provide necessary control functions, such as clock pulses, test modes, test control data, and etc. to the integrated circuit. The remaining signal interface pins are used to transfer data into and out of the data storage circuits contained in the integrated circuit.
At times, a signal interface or input/output pin, used to transfer data into or out of the data storage circuits, will function correctly in a direct current (DC) mode, that is, it will carry the proper current but will have resistive, inductive or capacitive aspects that affect the alternating current (AC) characteristics, i.e., the rise and fall times of signals passing through the pin. These AC characteristics, by delaying or skewing the shift time of these signals, causes the rise or fall time of the signals to be altered, i.e., to be slower than that called for in the circuit specification. To assure the final quality of the integrated circuit such AC defects must be ascertained by rigorous and extensive testing of the integrated circuit.
At the present time, such AC defect testing requires the use of high frequency automated test equipment (ATE) that provides a tester contact for each signal interface pin on the integrated circuit, i.e., for both functional circuit pins and data storage pins.
Testers having sufficient test contacts for large integrated circuits are expensive to purchase and newer integrated circuits are now being introduced with even higher storage data capacity and thus more input/output pins. The introduction of integrated circuits with more input/output pins means that manufacturers must either upgrade their old testers or purchase new testers.
Thus as integrated circuits continue to increase in pin count and become faster, it becomes increasingly expensive for manufacturers to provide automated test equipment that can speedily test the newer, larger signal pin count, integrated circuits.
Therefore, there is a compelling economic reason to extend the life of the present testers by providing a way to utilize presently available test equipment to test the newer, larger storage capacity, integrated circuits.
It also becomes desirable that means be found to test the newer, large storage capacity, integrated circuits with presently available testers.